I. Field of the Invention
The present invention relates to voice and data communication. More particularly, the present invention relates to a novel and improved method and apparatus for transmitting packetized voice traffic and data traffic over wireless communication networks.
II. Description of the Related Art
A modern day communication system is required to support a variety of applications. One such communication system is a code division multiple access (CDMA) system that conforms to the “TIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,” hereinafter referred to as the IS-95 standard, or a CDMA system that conforms to the “TIA/EIA/IS-2000-2 Physical Layer Standard for cdma2000 Spread Spectrum Systems,” hereinafter referred to as the IS-2000 standard. Another CDMA standard is the W-CDMA standard, as embodied in 3rd Generation Partnership Project “3GPP” Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214. A CDMA system allows for voice and data communications between users over a terrestrial link. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS”, and U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM”, both assigned to the assignee of the present invention and incorporated by reference herein. Other examples of communication systems are time division multiple access (TDMA) systems and frequency division multiple access (FDMA) systems.
In this specification, base station refers to the hardware with which the remote stations communicate. Cell refers to the hardware or the geographic coverage area, depending on the context in which the term is used. A sector is a partition of a cell. Because a sector of a CDMA system has the attributes of a cell; the teachings described in terms of cells are readily extended to sectors.
In a CDMA system, communications between users are conducted through one or more base stations. A first user on one remote-station communicates to a second user on a second remote station by transmitting data on the reverse link to a base station. The base station receives-the-data and can route the data to another base station. The data is transmitted on the forward link of the same base station, or a second base station, to the second remote station. The forward link refers to transmission from the base station to a remote station and the reverse link refers to transmission from the remote station to a base station. In IS-95 and IS-2000 FDD mode systems, the forward link and the reverse link are allocated separate frequencies.
Given the growing demand for wireless data applications, the need for very efficient wireless data communication systems has become increasingly significant. The IS-95 and IS-2000 standards are capable of transmitting data traffic and voice traffic over the forward and reverse links. A method for transmitting data traffic in code channel frames of fixed size is described in detail in U.S. Pat. No. 5,504,773, entitled “METHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION,” assigned to the assignee of the present invention and incorporated by reference herein.
A significant difference between voice traffic services and data traffic services is the fact that the former imposes stringent maximum delay requirements. Typically, the overall one-way delay of speech traffic frames must be less than 100 msec. In contrast, the delay of data traffic frames can be permitted to vary in order to optimize the efficiency of the data communication system. Specifically, more efficient error correcting coding techniques, which require significantly larger delays than those that can be tolerated by voice traffic services, can be utilized. An exemplary efficient coding scheme for data is disclosed in U.S. Pat. No. 5,933,462, entitled “SOFT DECISION OUTPUT DECODER FOR DECODING CONVOLUTIONALLY ENCODED CODEWORDS,” filed Nov. 6, 1996 and issued Aug. 3, 1999, assigned to the assignee of the present invention and incorporated by reference herein.
Another significant difference between voice traffic and data traffic is that the former requires a fixed and common grade of service (GOS) for all users. Typically, for digital systems providing voice traffic services, this translates into a fixed and equal transmission rate for all users and a maximum tolerable error rate for the speech traffic frames. In contrast, because of the availability of retransmission protocols for data traffic services, the GOS can be different from user to user and can be varied in order to increase the overall efficiency of the data communication system. The GOS of a data traffic communication system is typically defined as the total delay incurred in the transfer of a predetermined amount of data.
Yet another significant difference between voice traffic services and data traffic services is that the former requires a reliable communication link which, in the exemplary CDMA communication system, is provided by soft handoff. Soft handoff results in redundant transmissions from two or more base stations to improve reliability. However, this additional reliability is not required for data traffic transmission because the data packets received in error can be retransmitted. For data traffic services, the transmit power used to support soft handoff can be more efficiently used for transmitting additional data.
Various protocols exist for transmitting packetized traffic over packet-switching networks so that information arrives at its intended destination. One such protocol is “The Internet Protocol,” RFC 791 (September, 1981). The internet protocol (IP) breaks up messages into packets, routes the packets from a sender to a destination, and reassembles the packets into the original messages at the destination. The IP protocol requires that each data packet begins with an IP header containing source and destination address fields that uniquely identifies host and destination computers. The transmission control protocol (TCP), promulgated in RFC 793 (September, 1981), is responsible for the reliable, in-order delivery of data from one application to another. The User Datagram Protocol (UDP) is a simpler protocol that is useful when the reliability mechanisms of TCP are not necessary. For voice traffic services over IP, the reliability mechanisms of TCP are not necessary because retransmission of voice packets is ineffective due to delay constraints. Hence, UDP is usually used to transmit voice traffic.
Additionally, since voice traffic is time-sensitive, another protocol for delivering time-sensitive traffic is required. Real-time Transport Protocol (RTP), promulgated in RFC 1889, uses sequence information to determine the arrival order of packets and uses time-stamping information correct misalignments in the interarrival packet time, which is referred to as jitter. Jitter can be considered as the difference between the time when a packet is expected and the time when the packet actually arrives. An RTP header is used in conjunction with UDP and IP headers to provide voice traffic services over packet switching networks. The combined IP/UDP/RTP header is 40 bytes long, which is a significant percentage of the actual IP packet. In a slow communication link, the overhead required for transmitting IP/UDP/RTP headers may be unacceptable for end users.